The present invention relates generally to gas turbine engines, and, more specifically, to rotor cavity purging.
In a gas turbine engine, air is pressurized in stages in a multi-stage compressor and mixed with fuel in a combustor for generating hot combustion gases which flow downstream through several turbine stages. A high pressure turbine (HPT) includes a turbine nozzle at the combustor exit which channels the combustion gases between the HPT rotor blades which extract energy therefrom for powering the compressor.
The HPT may include a second stage with a corresponding turbine nozzle disposed downstream of the first stage blades followed in turn by a row of second stage turbine blades which extract additional energy from the combustion gases for also powering the compressor. A low pressure turbine typically follows the HPT and extracts further energy for powering a fan upstream of the compressor which produces propulsion thrust for powering an aircraft in flight.
As the combustion gases flow downstream through the turbine stages the pressure thereof is decreased as energy is extracted therefrom. Accordingly, an inter-stage seal is provided radially inboard of the second stage turbine nozzle to prevent the higher pressure combustion gases upstream of the nozzle from bypassing the nozzle to the second stage turbine blades. The seal is defined in part by a seal ring having forward and aft ends defining blade retainers attached to the corresponding rotor disks of the first and second stage blades.
An axially intermediate portion of the seal ring includes radially extending seal teeth which cooperate with an annular seal pad, typically in the form of honeycomb, which is attached to an inner band of the second stage nozzle. The seal teeth and pad define a labyrinth seal which provides a substantial flow restriction against the flow of air or combustion gases therethrough.
The forward portion of the seal ring defines with the first stage turbine rotor and the forward portion of the second stage nozzle inner band an upstream or forward cavity, and the aft portion of the seal ring defines with the aft portion of the inner band and the second stage turbine rotor a downstream or aft annular cavity.
The seal ring rotates with the first and second stage rotors and is subject to being heated therewith during operation. The forward and aft rotor cavities are typically purged with a source of cooling air during operation for reducing the temperature thereof for maintaining an effective useful life of the adjoining rotor components. The cavity purge air is typically provided by using compressor bleed air which first cools the vanes of the second stage nozzle and then is discharged into the forward and aft cavities for purging thereof.
The second stage vanes typically include corresponding perforated impingement baffle inserts therein which receive the cooling air from the compressor for impingement against the inner surface of the vane for vane cooling. The spent impingement air is then discharged through the nozzle inner band through respective forward and aft purge air holes disposed in flow communication with the respective forward and aft rotor cavities.
The post-impingement purge air has limited cooling capability for the rotor cavities since its temperature has increased substantially due to the impingement cooling of the vanes, and it has a reduced pressure available to drive it through the purge holes. Since the forward cavity is at a higher pressure than the aft cavity, less of a differential pressure with the supplied purge air is available in the former as compared with the latter.
Accordingly, the forward purge holes are typically not tangentially inclined through the inner band in view of the lack of available differential pressure, which results in increased temperature rise thereof due to windage thereof in the forward cavity as the first stage rotor rotates relative to the stationary inner band. However, a greater driving pressure differential is available in the aft cavity and therefore the aft purge holes may be tangentially inclined to direct the purge air tangentially toward the rotating second stage rotor for reducing windage losses and reducing the increase in temperature of the purge air.
Since the vane cooling air is obtained by bleeding a portion of compressor air at an intermediate stage upstream of the compressor exit its supply pressure is limited, and its differential pressure with the sink pressure in the rotor cavities decreases as engine speed and temperatures increase. The purge air to the rotor cavities is therefore reduced when it is needed the most.
Accordingly, the structural and functional operating characteristics of the rotor cavities limit the available cooling thereof which correspondingly limits the power potential of the engine over a suitable useful life.
One modification placed in commercial service to address the limited rotor cavity cooling includes a single dump hole added to the bottom of the impingement baffle in each vane near the trailing edge thereof. The dump hole provides pre-impingement air from the baffle directly into a common cavity feeding both the forward and aft purge holes. This modification has demonstrated in over a year of service limited improvement in rotor cavity cooling, at the expense of vane impingement cooling.
Accordingly, it is desired to provide a turbine nozzle having improved purge air cooling of the forward and aft rotor cavities.